1. Field of the Invention
The present invention relates to an electrical conductivity measuring system for a material, particularly relates to the electrical conductivity measuring system on a noncontact basis, in which a microwave is used.
2. Description of the Prior Art
Measurement of the electrical conductivity is essential in quality control and performance evaluation of a silicon wafer which is a typical material used for electronic devices.
A four-point probe method is conventionally adopted as the measuring method for the electrical conductivity of the silicon wafer. Though the four-point probe method has an advantage of the quite easy measurement, there is a problem that a leading end of the probe hurts a surface of the wafer because it is necessary that the leading end of the probe is in contact with the wafer during the measurement. Calibration of a thickness of the wafer is also required in quantitative evaluation. Therefore, the four-point probe method can not be adopted for on-line inspection in a manufacturing process of the silicon wafer or non-destructive inspection prior to the manufacturing of the device, and a spot check of few pieces of the silicon wafer products is the only way to inspect the silicon wafer.
The noncontact measuring method for the electrical conductivity is required to realize the non-destructive inspection. In the noncontact measuring method for the electrical conductivity, there is a technique in which a coil is used. However, a change in impedance of an induction coil depends on not only the electrical conductivity of the wafer but also permittivity and the thickness of the wafer. Accordingly, an additional measurement, e.g. the measurement in which the thickness of the wafer is determined by other technique is required in the quantitative evaluation of the electrical conductivity in which the coil is used. The technique using the coil also has the problem of low spatial resolution, so that it is not suited for the non-destructive inspection of the wafer prior to the manufacturing of the device.
In recent years, a study concerning the non-destructive evaluation of the semiconductor wafer using the microwave receives attention as the new measuring method on a noncontact basis. The measurement of a lifetime of a minority carrier (see non-patent reference 1) has been realized, and the evaluation of photoconductivity of the wafer (see non-patent reference 2) and dependence on the electrical conductivity of an absorption peak of the microwave (see non-patent reference 3) have been also reported.
However, the quantitative evaluation of the electrical conductivity of the semiconductor wafer has not been successful yet due to influence of the permittivity and thickness of the wafer, or the like.
(Non-Patent Reference 1)
J. Schmidt and A. G. Aberle: Accurate Method for the Determination of Bulk Minority-Carrier Lifetimes of Mono-and Multicrystalline Silicon Wafers, Journal of Applied Physics, Vol. 81, No. 9, pp. 6186-6199, (1997)
(Non-Patent Reference 2)
J. R. Niklas, W. Siegel, M. Jurisch and U. Kretzer: GaAs Wafer Mapping by Microwave-Detected Photoconductivity, Materials Science and Engineering B, Vol. 80, No. 1-3, pp. 206-209, (2001).
(Non-Patent Reference 3)
R. M. Lewis and J. P. Carini: Frequency Scaling of Microwave Conductivity in the Integer Quantum Hall Effect Minima, Physical Review B, Vol. 64, No. 7, 073310, (2001).